Commercially, it is highly desirable for an electronic display (e.g., a liquid crystal display) to be as thin and light as possible while still maintaining a high degree of ruggedness to withstand forces resulting from pressure, decompression, shear, and shock. In the area of mobile electronics, such as cell phones and personal digital assistants (PDAs), size and weight are critical factors to the commercial success of a product, but currently breakage of the displays within these devices remains the primary cause of repairs and product returns.
In addition, the need for electronic displays that can actually be bent has been acknowledged in several areas. So-called ‘electronic paper’ in which fiber paper is replaced with a display would be much more compelling as a product if the electronic display could be rolled up or folded like traditional paper. Wearable electronics, such as computers or multifunction watches, would be much more comfortable to the wearer if the display were conformable. Chip cards, which have strict flexure life-test performance standards, would be able to incorporate flexible displays and still conform to those standards.
One approach to achieving the desired rugged and flexible display is to substitute polymer (or other flexible) substrates for the glass substrates conventionally used. Achieving such a substitution of polymer for glass has been an area of active research within the display community for a number of years.
Polymer substrates have the property of not springing back into their original shape, as do glass substrates, following flexure of the display. This can result in a change in the separation between the two substrates, with resulting degradation of display quality. A solution to this problem is providing a structural bond between the polymer substrates. A structural bond is able to maintain the spacing by keeping the polymer substrates engaged against intervening spacing elements (e.g., tiny glass spheres or fibers) even during flexure of the display.
Display contrast is another area in which rugged and flexible displays have had difficulty matching the performance of conventional displays. One technique for increasing display contrast is to give the liquid crystal molecules a homeotropic alignment (i.e., the optical axis, or long dimension, of the molecules is generally perpendicular to the faces of the substrates) in the absence of an electric field. This gives the display the highest possible contrast because in the off state a homeotropic cell placed between crossed polarizers has extremely low transmission. In other words, the dark state is as dark as it can be (and depends on the extinction ratio of the polarizers used). When an electric field is applied to the liquid crystal in a selected area of the cell, the molecules in that area deviate from the homeotropic alignment towards an oblique or planar alignment, in which the molecules assume an orientation generally oblique and even parallel to the substrate (perpendicular to the substrate normal).
There are various known ways of achieving homeotropic alignment. The most common technique is to provide an alignment layer of, e.g., lecithin, on one or both of the substrates. Other alignment techniques have been attempted. E.g., a pre-manufactured layer of filter material with tiny pores (in which the molecules are received) can be inserted between the substrate faces. An electric field can be applied during manufacturing to position the molecules in homeotropic orientation, while slender tendrils of polymer are polymerized between the molecules to hold them in the desired alignment (sometimes referred to as a “polymer stabilized” display). But none of these approaches provided a structural bond between the substrates, and thus are ineffective for achieving a rugged or flexible display.
An early effort at providing a structural bond between substrates was the polymer dispersed liquid crystal (PDLC) display, in which the liquid crystal molecules were dispersed within a polymer matrix. After assembling the display, the polymer was cured, typically by ultraviolet light. During the polymerization the liquid crystal separated out from the polymer into microscopic droplets. The polymer provided a structural bond between the substrates. As the droplets of liquid crystal were not in contact with either substrate face, an alignment layer could not be used to orient the molecules, and the displays were operated in a different and less desirable “scattering mode”. Examples of PDLC displays and related technology are U.S. Pat. Nos. 4,688,900, 5,321,533, 5,327,271, 5,434,685, 5,504,600, 5,530,566, 5,583,672, 5,949,508, 5,333,074, and 5,473,450.
An improvement on the PDLC display was the Phase Separated Composite Organic Film (PSCOF) display (described in U.S. Pat. No. 5,949,508) in which the liquid crystal and polymer were disposed near opposite substrates, with widely separated support polymer dots extending fully across the gap between the faces. The dots provided an effective structural bond between the substrates and because in most locations one substrate face was exposed to the liquid crystal molecules, an alignment layer could be provided on one of the substrates. Typically, the alignment layer in PSCOF display positions the liquid crystal molecules in a homogeneous alignment (optical axis generally parallel with substrates), and the molecules are rotated to a homeotropic alignment by the presence of an electric field.
In both PDLC and PSCOF displays, the concentration of polymer was from 20 to 80 percent of the weight of the mixture of polymer and liquid crystal. In the polymer tendril (polymer stabilized) display, in which slender tendrils were formed while the molecules were held in homeotropic alignment by an applied electric field, the concentration of polymer was much less, e.g., about 3-5 percent by weight of the mixture of polymer and liquid crystal.
Other patents with potentially relevant background are: Rosenblatt et al., “Chiral Nematic Liquid Crystal with Homeotropic Alignment and Negative Dielectric Anistropy” (U.S. Pat. No. 5,477,358); Anderson et al., “Vertically Aligned Pi-Cell LCD having On-State with Mid-Plane Molecules Perpendicular to the Substrate” (U.S. Pat. No. 6,067,142); Patel, “Inverse Twisted and Super-twisted Nematic Liquid Crystal Device” (U.S. Pat. No. 5,701,168); Ogishima et al., “Liquid Crystal Display Device with Homeotropic Alignment in which Two Liquid Crystal Regions on the Same Substrate Have Different Pretilt Directions Because of Rubbing” (U.S. Pat. No. 5,757,454); Rosenblatt et al., “Cholesteric Liquid Crystal Devices” (U.S. Pat. No. 5,602,662); Kaufmann et al., “Homeotropic Nematic Display with Internal Reflector” (U.S. Pat. No. 4,492,432).